to be fucked in the ass, forcefully and unwillingly. the victim can be a man or woman, but the ass raper is always a dude.

prison guy #1:oh shit, there goes my soa...AHHHHH FUCK YOU
prison guy #2: shut up bitch and take it like a man
prison guy #3: dude, check out that guy getting ass raped,
he should have had a rope on that soap.
prison guy #4: i wish i could get some action

in it's sinister form, means a depraved and criminal sex act. Rape is any kind of sexual act performed on someone against their will.
However, it has become quite popular slang in some online gaming communities, usually just amongst lads (so there's less chance of someone being offended by its use) to mean, usually, beaten hands down. Sometimes used more when you are beaten in an unlucky situation.

Did you see that? there were three of them left against just me! i was assraped!